Various forms of optical adapters and converters are known that permit objective lenses designed for a certain size image format to be utilized for imaging on another size image format. However, most of the known rear lens attachments which are designed to be placed in a converging beam increase the image size instead of reducing it. Examples of such rear attachments are disclosed in U.S. Pat. Nos. 4,913,537; 4,840,466; 4,394,071; and 4,422,734.
U.S. Pat. Nos. 4,830,474; 4,634,235 and 4,264,151 disclose optical adapters which decrease the image size when placed behind the objective lens. However, these adapters are not suitable for SLR cameras because while decreasing the focal length of the entire lens system, they are also decreasing its back focal length (BFL) and thus leave no room for an SLR camera mirror. In addition, they can not be used in other applications that require a long back focal length.
U.S. Pat. Nos. 4,862,293; 4,814,811; 4,823,199; and 4,953,029 describe lens systems in which an optical adapter includes a re-imaging lens system and a CCD array which are placed behind the film plane of a 135 format SLR camera. However, when this type of adapter is used, the camera body becomes thick and the viewfinder must be modified to provide comfortable viewing.
Thus, there is a need to provide a system in which the camera body could be kept compact (or in which the original camera body could still be used) and in which the viewfinder does not have to be modified or re-designed.